1. Field of the Invention
The present invention relates to a high DC gain wideband operational amplifier operable at a low operating voltage.
2. Description of the Related Art
A prior art operational amplifier is constructed by an input circuit including a differential pair formed by N-channel MOS transistors associated with an N-channel MOS transistor serving as a tail current source and load N-channel MOS transistors, and output circuits connected to the input circuit, each of the output circuits including a drive cascode circuit and a load cascode circuit (see: T. B. Chuo et al., “A 10b, 20 Msample/s, 35 mW Pipeline A/D Converter”, IEEE, Journal of Solid-State Circuits, Vol. 30, pp. 166-172, March 1995, and M. Waitari et al., “A 220-MSample/s CMOS Sample-and-Hold Circuit Using Double-Sampling”, Analog Integrated Circuits and Signal Processing, 18, pp 21-31, 1999). This will be explained later in detail.
In the above-described prior art operational amplifier, however, the minimum operating voltage is relatively high so that the power consumption would be increased.
In order to decrease the minimum operating voltage, the load N-channel MOS transistors of the input circuit may be replaced by non-doped N-channel MOS transistors whose gate-to-source voltage is about 0.1 V; in this case, however, the gate-to-source capacitance of the non-doped N-channel MOS transistors is very large, which would remarkably degrade the bandwidth characteristics.
Also, in the above-described prior art operational amplifier, a high DC gain cannot be realized, since the input circuit adds a non-dominant pole to the transfer function, so that the gain of the input circuit must be kept low enough to ensure that this non-dominant pole lies at a sufficiently high frequency, so that the DC gain is subject to only the output circuits.
Note that, in order to increase the DC gain, triple cascode circuits may be used instead of the above-mentioned cascode circuits, in this case, however, the dynamic output range would be decreased. Also, in order to increase the DC gain, gain boost circuits can be connected to the above-mentioned cascode circuits; in this case, however, the integration would be degraded and the power consumption would be increased. Regarding “gain boost circuits”, refer to FIG. 6 of T. B. Chuo et al., “A 10b, 20 Msample/s, 35 mW Pipeline A/D converter”, IEEE Journal of Solid-State Circuits, Vol. 30, No. 3, pp. 166-172, March 1995.